Producing a microscopic contiguous free-stream liquid jet using a solid-walled convergent nozzle (so called “Rayleigh source”) and break-up of the liquid jet into a linear stream of almost monodisperse droplets (so called “Rayleigh-Plateau break-up”) are generally known. The ability to trigger the Rayleigh-Plateau break-up by imposing a dominant acoustic drive signal has been documented e. g. by U. Weierstall et al. in “Exp. Fluids” vol. 44, 2008, p. 675-689. The triggered break-up results in a perfectly periodic, monodisperse, and linear stream of droplets. Either the contiguous section of the liquid jet or the droplet stream can be employed for experimental/process use, depending on the demands of the experiment/process at hand. However, Rayleigh sources may have disadvantages in terms of clogging of the solid-walled convergent nozzle, making it impossible to reliably generate liquid jets of smaller than about 20 μm in diameter (yielding droplets of 40 μm diameter). This is far too large e. g. for X-ray scattering experiments with biological samples suspended in liquids. Furthermore, the large jet diameter results in an undesirably large sample consumption.
Furthermore, a microscopic contiguous linear liquid free-stream jet can be produced using a sheath gas stream (see e. g. A. M. Gañán-Calvo in “Phys. Rev. Lett.” vol. 80, 1998, p. 285-288: or U.S. Pat. No. 8,272,576). The sheath gas stream is provided by a so-called gas dynamic nozzle (or: Gas Dynamic Virtual Nozzle, GDVN) as disclosed in U.S. Pat. No. 8,272,576. The gas dynamic nozzle includes an inner tube carrying a liquid, an outer tube carrying the focussing sheath gas, an exit channel and an exit aperture. As with Rayleigh sources, the contiguous liquid jet presents a Rayleigh-Plateau break-up into a linear stream of droplets (see e. g. D. P. DePonte et al, in “J. Phys. D: Appl. Phys.” vol. 41, 2008. p. 195505-195512).
With the gas dynamic nozzle technique, the liquid jet is continuously formed with a “virtual nozzle” created by the convergent sheath gas rather than by a convergent solid-walled nozzle. As a result, the GDVN injectors are much less susceptible to clogging. Furthermore, GDVN injectors may be used to routinely produce liquid jets having a diameter of 5 μm (Yielding droplets of about 10 μm diameter after break-up, which is significantly smaller than the clogging-limited droplet size of a Rayleigh source). By placing the exit of the timer tube very nearly at or even beyond the end of convergent sheath gas flow, where the gas is actually expanding as a free-jet expansion, even smaller free-streams can be delivered, e. g. having a diameter as small as 300 nm (600 nm droplets after Rayleigh break-up), see A. M. Gañán-Calvo in et al. in “small” vol. 6, 2010, p. 822-824.
Despite this jet diameter reduction, liquid jets conventionally produced with gas dynamic nozzles may have a disadvantage resulting from the continuous flow nature of the liquid jet and the resulting continuous substance consumption since, as an example, precious biological samples often are available in amounts measured in tens of μl only. If such biological samples suspended in a liquid are to be investigated by measuring X-ray scattering at a continuous flow liquid jet of the suspension and the liquid jet has a flow rate of about 10 to 20 μl/min the measuring time or number of measurements are strongly limited. This problem is even intensified by the fact that measurements often are conducted with pulsed probe beam sources. Large portions of the continuous flow liquid jets then cannot be utilized for the measurements as they are not hit by probe beam pulses.
As a further disadvantage of gas dynamic nozzles, it has been found that an irregular “dripping” behaviour of gas-focused liquid jets may occur (see e. g. J. M. Montanero et al. in “Phys. Rev. E” vol. 83, 2011, p. 036309: E. J. Vega et al. in “Phys. Fluids.” vol. 22, 2010, p. 064105; and T. Si et al. in “J. Fluid Mech.” vol. 629, 2009, p. 1-23, and in “Phys. Fluids” vol. 22, 2002. p. 112105). The dripping behaviour is an undesirable mode of operation as samples are provided in an irregular and non-reproducible fashion. Therefore, the above investigations have been conducted for characterizing the transition from the dripping to the jetting mode of the nozzle in order to avoid the dripping behaviour.
Another conventional technique of delivering small amounts of liquids for a measurement uses “droplet-on-demand” injectors (DoD injectors), which are operated in vacuum. However, these DoD injectors have a disadvantage resulting from evaporative cooling in vacuum, causing the nozzle to freeze shut between droplets. Heating the DoD injector nozzle to prevent freezing is problematic for most biological molecules, which invariably denature at just above body temperature.